CHAPTER 2: LITERATURE REVIEW
2.6 Case Study 3: Solar Absorption Cooling System in Brazil (2010)
In Brazil the energy demand for refrigeration and air-conditioning correspond to approximately 15 % (134 TWh/year) of the total country energy use. Around 48% of energy is consumed in commercial and public buildings due to air conditioners, usually by driving electrical vapour compression chillers [48].
15 The object of the case study is the intended auditorium at the UNESP University in Guaratingueta, which is likely to be equipped with a solar cooling system. The solar radiation in Guaratingueta lies around 5.5 kWh/m² in between. Guaratingueta lies between Sao Paulo and Rio de Janeiro in the Brazilian Megalopolis. Before the economic feasibility can be calculated the acquisition and operation cost must be investigated. Below are actual cost tables (Tables 2.1 and 2.2) for the applicable solar- assisted air-conditioning system and Split Air conditioning system [48].
Table 2.1: Acquisition and specific costs per kW cooling capacity for two different system combinations [48].
16 Table 2.2: Comparison of electricity consumption and operation costs of a solar-assisted air conditioning system and electrically driven compression vapour split air conditioning system.
[48]
The next two Figures 2.1 and 2.2 shows the different acquisition and operation costs for a solar- assisted air conditioning system against a split air conditioning unit system in two countries, namely Guaratingueta and Minas Gerais. Besides the shown cost development due to the specific electricity cost in Guaratingueta, it has presented the cost gradient through a higher electric energy price, which exists for example in Minas Gerais, where, as well, very good solar irradiance occurs.
There are no interest rates on the investment capital or maintenance cost considered, as well, no intended possible public subsidies and electricity cost elevation. If there is an interest rate of only 1.5
% per year of the investment cost of 191.147 R$ the payback-time would be 5 years longer. This means a payback period of around 21 years, thus the system would not be profitable during the system’s lifespan. The usual interest rate of such a credit in Brazil is around 8.5 % per year. Hence it is essential to become profitable with a very low interest rate, lower than 1.5 % per year.
17 In Brazil, as yet there is no subsidy or tax relief for those who exploit renewable energy. However, the Brazilian government just discussed a law (Lei 630/03) [48] which proposes financial support. In Germany there are several solar thermal energy incentives. For example the Reconstruction Loan Corporation (KfW) pays 30% of the solar cooling system investment, if the collector array is bigger than 40 m² [48].
Figure 2.1 Acquisition and operation cost of solar-assisted air-conditioning system and conventional split air-conditioning system in Guaratingueta [48].
Figure 2.2 Acquisition and operation cost of solar-assisted air-conditioning system and conventional split air-conditioning system in Minas Gerais [48].
18 Note: Operation costs are calculated with an electric price of 0.60 R$/kWh which is the price in the Brazilian State of Minas Gerais by CEMIG (companhia energetica de Minas Gerais) in Figure 2.1 and 2.2 graphs.
An important definition to evaluate the economic feasibility is the meaning of “critical operation time”, which is understood as the payback period of the capital cost of using a solar assisted air conditioning system. If the solar cooling system within the lifetime (here 20 years) will be longer in operation than the critical operation time, the high cost of acquisition pays off.
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CHAPTER 3: SOLAR THERMAL COOLING TECHNOLOGIES
This Chapter presents a description of the various solar cooling technologies available in the market today. The comparison of the Coefficient of Performance (COP) of the solar cooling technologies concludes the Chapter.
3.1 Types of Solar Thermal Cooling Technologies
The vapour absorption cycle uses two fluids and some quantity of heat input, rather than the electrical input as in the more familiar vapour compression cycle to achieve air conditioning. The vapour compression and absorption air conditioning cycles both achieve the removal of heat via evaporation of a refrigerant at a low pressure and reject heat through the condensation of the refrigerant at a higher pressure. The method employed to create the pressure difference and to circulate the refrigerant is the primary difference between the two cycles.
This cycle is interesting as it uses a renewable source of energy to drive the cooling process. Due to the nature of the solar resource as a renewable source of energy, it can be used for its thermal energy and for its solar radiation to produce electric power. Thus there are three broad categories of Solar Cooling Technologies based on the manner in which solar energy is utilized. Solar Electrical Cooling Technologies utilize the solar radiation for electricity by suitable conversion technologies, Solar Thermal Cooling uses the thermal energy from the solar radiation and the third category uses a combination of Solar Thermal and Electrical energy.
The use of ‘solar to electrical power’ cooling systems works on the principle of providing electricity from conversion of solar radiation to electricity via the photovoltaic cell storing electrical energy in a battery which is used to energize conventional mechanical compression cycle chillers. The combined solar thermal and electrical cycle systems can be of various types and are hybrid systems that use stored electricity captured from solar energy to minimize or eliminate the use of electrical energy from the grid for their operation. The solar thermal cooling technologies are classified according to their cycle operation, i.e. open, closed and thermomechanical as illustrated in Figure 3.1.
20 Figure 3.1: Solar thermal cooling technologies
The solar thermal cooling systems incorporate different setups and cycles that can be classified into 3 main groups, the Open Cycle, which uses the Liquid and Solid Desiccant systems, the Closed Cycle which is the Absorption and Adsorption cycle and the Thermomechanical System which uses the basic principle of the Ejector cycle.
The Ejector cycle is not a cooling solution that is available off the shelf. The Absorption, Adsorption, solid and liquid desiccant systems existing around the world in 2006 are represented in Figure 3.2. In 2006, much of the Solar Thermal Cooling market is dominated by the Absorption cooling system followed by the Adsorption Cooling Cycle and the Desiccant Cooling (DEC) liquid and solid systems are in the minority as can be seen in Figure 3.2. However, in Figure 3.3, which shows the solar driven chillers market share in 2008, the Adsorption Cooling systems are in the minority with the Desiccant Cooling (DEC) systems taking the second largest portion of the market share.
Figure 3.2: Solar Thermal Cooling Installations around the world in 2006 [8].
21 Figure 3.3: Solar Thermal driven chillers share in 2008 [8].
3.2 Closed Cycle Cooling
Closed loop cycles produce chilled water in thermally driven chillers for which any type of distribution system can be employed to circulate the cold between working fluids. These chillers are called absorption and adsorption chillers, with absorption chillers being the more popular option. A thermal compression of the refrigerant is achieved by using a liquid sorbent/solution and a heat source, thus replacing the electric power consumption of a mechanical compressor.
3.2.1 Absorption Cooling
The principle of operation of this type of cooling is explained in detail in Chapters 5 and 7 that follow.
The main components of an absorption chiller are the generator, condenser, absorber and evaporator as shown in Figure 3.4.
Figure 3.4: Main components of an absorption chiller [49].
22 The cooling effect is achieved by the evaporation of the refrigerant, such as water in the evaporator at a very low pressure. The vaporized refrigerant is absorbed in the absorber, thereby diluting the liquid sorbent, such as Lithium Bromide solution, ammonia or Lithium Chloride. The solution is pumped to the generator where the solution is regenerated by the application of driving heat (such as hot water).
The refrigerant leaves the solution and condenses on the condenser by the circulation of cooling water and is circulated back to the evaporator through the expansion valve.
This type of cooling uses two working liquid pairs that have an affinity toward each other such as water and Lithium Bromide (H2O/LiBr), water and Lithium Chloride (H2O/LiCl) and water and ammonia (H2O/ NH3). The most popular chillers used for space cooling are the chillers that use the H2O/LiBr working pair whereas the H2O/NH3 working pair is used mainly for refrigeration. The process of absorption cooling will be covered in detail in Chapter 4 and Chapter 6. The cycles can employ double effect refrigeration stages to achieve greater COPs.
There are many different manufacturers of solar absorption chillers and their market captured out of 280 installations is represented in Figure 3.5.
Figure 3.5: Absorption chiller brand market share out of 280 installations, 2008 [8].
23 3.2.2 Adsorption Cooling
In the adsorption chiller, instead of liquid sorbents, a solid sorbent is employed and water is used as a refrigerant. The adsorption process incorporates two sorbent compartments indicated by 1 and 2 in Figure 3.6, one of which is the evaporator and the other the condenser. While the sorbent, typically silica gel, is regenerated in the first compartment using an external heat source, typically hot water, the sorbent in the second compartment adsorbs the water vapour entering from the evaporator.
Cooling water circulated in compartment 2 ensures a continuous adsorption. The water in the evaporator is changed into a gas phase causing the cooling effect.
Figure 3.6: Adsorption Cycle [49]
The driving temperatures required for adsorption chillers are usually around 80C, with an acceptable temperature as low as 60C, however their typical COPs are lower than that of absorption chillers, and that being 0.6. The major commercial manufacturers of these chillers are Japanese manufacturers, Nishyodo (70kW to 500kW) and Maekawa (50kW to 350kW),
3.3 Open Cycle Cooling: Desiccant Cooling Systems
This type of cooling involves the direct cooling/treatment of ambient air. Desiccant Cooling Systems use water as a refrigerant in direct contact with air. The cycle is a thermally driven process that involves the combination of evaporative cooling with air dehumidification by a desiccant which is a hygroscopic material. The refrigerant after providing the cooling effect is discarded from the system and replaced with new refrigerant, hence the term open cycle. This type of system uses silica gel or Lithium Chloride as sorption materials on a rotating wheel.
24 Warm, humid ambient air enters the slowly rotating desiccant wheel and is dehumidified by the adsorption of water as depicted in process 1-2 in Figure 3.7. This air then passes through a heat recovery wheel where the air is precooled (2-3). The air is then conditioned to a desired temperature and humidity between process 4-5 and 3-4 respectively. The return air or air removed from the rooms is humidified close to saturation point (6-7) for maximum cooling potential to enable an effective heat recovery (7-8). The desiccant wheel is finally regenerated by the application of external heat (50C to 70C) to allow continuous operation of the dehumidification process. This system can also be modified for space heating modes.
Figure 3.7: Solid Desiccant Cooling [49].
Liquid desiccant cooling systems, utilize water/lithium chloride solution as sorption materials to dehumidify the air. This type of system shows several advantages over solid desiccant cooling systems as they achieve a higher air dehumidification at the same driving temperature and will be better suited to areas that have high humidity.
3.4 Thermomechanical Cooling: The Ejector Cycle
The ejector cycle works on the principle illustrated in Figure 3.8. This system uses a generator powered by low grade heat, evaporator condenser, expansion device, and ejector and circulating pump. Low grade heat (Qb) at around 80C is used to evaporate high pressure liquid refrigerant into vapour (process 1-2) which enters the ejector and is referred to as the primary fluid and accelerates through the nozzle.
25 The reduction in pressure that results induces vapour from the evaporator at point 3, known as the secondary fluid. The primary and secondary fluids mix and enter the diffuser section where the flow decelerates and pressure recovery occurs. The mixed fluid then rejects heat to the surroundings (Qc) upon being condensed. Some of the fluid flowing out of the condenser at point 5 is pumped to the boiler for the completion of the cycle whereas the rest of the liquid is expanded through the expansion device. This then enters the evaporator at point 6 as a mixture of liquid and vapour. Evaporation occurs producing a cooling effect (Qe) and the resulting vapour is then drawn into the ejector at point 3. The refrigerant which is the secondary fluid mixes with the primary fluid in the ejector and is compressed in the diffuser section before entering the condenser at point 4. The cycle is then repeated.
Figure 3.8: Ejector cooling cycle
Due to the low COP (0.2-0.3) of such a system, it is not commercially available off the shelf and requires a sufficient availability of low grade heat.
3.5 Comparison of Solar Thermal Cooling systems
The cooling technologies that use solar thermal energy each has its advantages and disadvantages based on the materials being used and the type of cycle employed. The Absorption cycle using the H2O/LiBr working pair is the most successful on the commercial market due to its high thermal COP in comparison to the other technologies as seen in Figure 3.9. The thermal COP is the ratio of the cooling capacity of the system to the heating power delivered to the system by solar collectors directly or indirectly through storage vessels.
26 The heat rejection medium temperature usually gives a good indication of what the COP of the thermal driven system is and this can be seen in Figure 3.10. The higher the heat rejection medium temperature, which is the cooling water, the lower the COP of the system represented in Figure 3.9.
Figure 3.9: COP as a result of heating medium temperature [50].
Figure 3.10: COP as a result of heat rejection medium temperature [50].
27 One of the disadvantages of solar thermal cooling technologies is the high capital cost involved compared to mechanical vapour compression cooling technologies using the R134a refrigerant. The high capital cost is due to the cost of the solar collector array needed to capture the thermal energy that drives the absorption cooling cycle. An indication of the influence of the collector area needed to the capital cost can be seen in Figure 3.11.
Figure 3.11: Initial system cost as a function of the specific collector area [50].
Water and Lithium Bromide absorption chillers have a relatively average initial cost compared to other solar thermal powered cooling cycles. The collector area needed for optimum operation of chillers depends on the solar insolation received at the location that it is installed. The thermal coefficient of performance of solar thermal based cooling cycles can be seen in Figure 3.12.
Figure 3.12: Annual thermal performance for the evaluated projects [50].
28 Although South Africa presently has one of the cheapest electricity costs in the world, solar thermal driven cooling cycles has not received much attention. However, due to the present energy crisis experienced by South Africa’s public utility provider, Eskom, these technologies are being considered. Chapter 4 investigates the energy usage in South Africa. Owing to the nature of solar energy, these technologies need to be proven in South Africa and hence the focus of the present study.
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CHAPTER 4: ENERGY IN SOUTH AFRICA
This Chapter gives the energy use in South Africa for the past years for primary energy supply and energy consumption per Gross Domestic Product (GDP). A snapshot of the energy use per sector is given together with the CO2 emissions and the overview of South Africa’s Electricity Supply Industry. The relationship between air conditioning and electricity usage is investigated and the potential for renewable energy in South Africa is given.
4.1 Energy Consumption in South Africa
South Africa has three groups of electricity generators, i.e. the national public electricity utility, Eskom, the municipal generators and the Auto-generators. The Auto-generators are industries which generate electricity for their own use and these include the pulp mills, sugar refineries, Sasol, Mossgas and metallurgical industries [51]. The country’s total primary energy supply can be seen in Figure 4.1.
Figure 4.1: Total Primary Energy Supply in South Africa [52]
It can be seen that South Africa relies heavily on coal, oil and biofuels and waste for its energy, of which coal, biofuels and waste is mainly used for energy production. This can be seen in Figure 4.2.
A significant portion used for electricity generation as depicted in Figure 4.3.
30 Figure 4.2: Energy Production in South Africa [52]
Figure 4.3: Electricity generation by fuel in South Africa [52]
The Gross Domestic Product (GDP) of South Africa had the 16th largest energy consumption in the year 2001. Figure 4.4 shows their GDP in relation to energy consumed in comparison with other countries in the world. One of the reasons for this is due to their nature of activities that dominate their economy. Mining, minerals processing, metal smelting and synthetic fuel production are
31 intensive users of energy. Figure 4.5 and Figure 4.6 show the final energy use by sector and final energy use by carrier respectively.
Figure 4.4: Energy Consumption per GDP per capita (2000) [53]
Figure 4.5: Energy Use by Sector, year 2000 [53]
Figure 4.6: Energy use by carrier, year 2000 [53]
Another reason for the high energy intensity is that there are instances where South Africa is wasteful in their use of energy. There was a lack of awareness during this time of energy efficiency, especially in the emission of greenhouse gases.
32 In the year 1999, South Africa relied almost completely on fossil fuels as a primary source of energy, i.e. 90%, with coal making up 75% of this Figure. South Africa is one of the main contributors to carbon dioxide emissions, which is the main greenhouse gas linked to climate change due to their combustion of coal to produce electricity. Figure 4.7 shows their ranking of CO2 emissions amongst the countries of the world in 1999. Their emissions per capita is close to half that of the United States of America, whereas their economic development is ranked “two worlds” behind the USA.
Figure 4.7: Carbon dioxide emissions per capita (IEA 2001) [54].
The economic sectors responsible for greenhouse gas emissions in South Africa can be seen in Figure 4.8.
Figure 4.8: Economic sectors responsible for greenhouse gas emissions in 2000 [51].
The “Energy Outlook for South Africa: 2002” [15] document released by the Eskom Energy Research Institute showss that there was a high growth in electricity demand during the 1960s and 1970s, where it was above 8%, in some years, followed by a drop in growth of demand to about 2% in the 1990s.
33 Due to the past large demands for electricity, Eskom had built large coal power stations which allowed them to have surplus capacity until 2008. Unfortunately Eskom had not built additional capacity to cope with the electricity demand increases after 2008 and the electricity consumers in the country that relied on energy from Eskom had to suffer periodic load shedding.
South Africa’s operating power stations providing electricity to the country cannot cope with the present demand as the installed electricity infrastructure has reached its full capacity. Planning should have occurred such that additional capacity was installed timeously to meet the power demand of a booming economy. Figure 4.9 and Table 4.1 shows Eskom’s installed and operating power stations and the projected demand respectively.
Figure 4.9: Eskom’s Electricity capacity [55]